David Cameron would like the world to believe he is a measured war leader who follows the letter of the law and avoids the gung-ho language of Margaret Thatcher.

So it came as a surprise to Conservative MPs, many of whom have severe doubts about the military action in Libya, when the prime minister turned up with a "secret weapon" at last night's meeting of the backbench 1922 committee.

With a great flourish, Arnold Schwarzenegger marched into committee room 14 of the House of Commons, which overlooks the Thames, to endorse the prime minister's leadership over Libya.

Schwarzenegger is an unlikely ally for a prime minister who wants to portray himself as a cautious war leader. In The Terminator Schwarzenegger walked into a gun store to order a weapon with the words: "The Uzi nine-millimetre."

At the end of his brief appearance before last night's meeting Schwarzenegger signed off with his most memorable line from the film. "I'll be back," he said, as he was escorted out of the meeting with bodyguards dressed in black tie as they headed to the Royal Albert Hall for Mikhail Gorbachev's 80th birthday party.

The former Californian governor told the Guardian that he wanted to support Cameron on Libya. "I was invited by the prime minister to say a few words," he said. "David is doing such a great job. It was very good to pump them up, to tell them they're doing a great job on Libya, a great job for Britain in making it live within its means and a great job with policies to protect the environment."

Schwarzenegger was impressed by his surroundings. "It was such a historic room, they have been holding these meetings since 1922," Schwarzenegger said of committee room 14 where he addressed the 1922 committee under a giant painting of Gladstone's 1868 cabinet by Lowes Cato Dickinson. Within two years of the painting British foreign policy was dominated by the Franco-Prussian war. But on that occasion Britain remained neutral.

Downing Street was delighted by Schwarzenegger's appearance, which came amid mounting concerns among Tory MPs that the government may be prepared to arm rebels in Libya. Cameron, who usually addresses the 1922 committee in December and July, was invited to attend the meeting to address the concerns of backbenchers.

One senior No 10 source said: "Arnie is David's secret weapon. We can't arm the rebels but at least we have Arnold Schwarzenegger."

The former governor hailed Cameron as a centreground leader as he quoted Eisenhower: "I despise people who go to the gutter on either the right or the left and hurl rocks at those in the centre."